Modern high performance disk drives employ head positioner servo loops. The function of the head positioning servo system within the drive is two-fold: first, to position the read/write head over a data track with sufficient accuracy to enable reading and writing of that track without error; and, second, to position the write element with sufficient accuracy not to encroach upon adjacent tracks to prevent data erosion from those tracks during writing operations to the track being followed. In order to satisfy these requirements, the tracking system must be designed to reject disturbances. These disturbances include noise from sources such as spindle bearings, air turbulence, etc., and can be classified into two general categories, those that generate repeatable runout (RRO) and those that generate non-repeatable runout (NRRO). The term "repeatable" is used to describe periodicity on a revolution-by-revolution basis as opposed to a track-by-track basis. The response of the head positioning servo system to the RRO and NRRO sources is track misregistration (TMR). TMR can be classified into two groups, write-to-read, and write-to-write. Write-to-read TMR is the difference in the head trajectory during a write pass and any subsequent read pass of a data track (overwrite and readback ability). Write-to-write TMR is the difference in the head trajectory during a write pass and a subsequent trajectory during a write pass on a neighboring track (data erosion). This invention addresses a method of minimizing the RRO components that are a major source of write-to-write TMR.
Embedded servo systems derive head position information from servo information interspersed within the data blocks written on a surface of a rotating magnetic disk. One advantage of employing embedded servo information is that the same head and electronics is used to read both user data and head position information. One of the major sources of RRO is the servo write process that occurs during disk drive manufacturing. During servo write, position information is written within each data track as embedded servo sectors. This position information, usually in the form of a pattern of circumferentially sequential, radially offset servo bursts within each servo sector, nominally defines the center of the track. This servo information must be written with great fidelity as it is read back by the read element of the head during drive operation to determine the position of the head relative to the data track. The NRRO disturbance (bearing noise, air turbulence, servo writer vibration, etc.,) that occurs during servo write is essentially frozen into the written position information and becomes the RRO for the particular track.
Once a servo pattern has been written, the resulting Position Error Signal (PES) can be measured and the RRO for each servo sample calculated. The resulting RRO values can then be written back into each servo sample in a digital format. (Analog burst trimming by overwrite is still subject to NRRO). There are two opportunities to determine the RRO: at the servo writer station during servo writing, and "off line" during subsequent drive self testing following the servo write process.
Measuring and writing the resulting RRO correction on the servo writer is reasonably straight forward. However, one disadvantage arising from using the servo writer for this purpose is the time required for, and therefore the cost associated with, using the servo writer station. Embedded servo sector burst patterns are typically written in a clean room environment with the aid of a laser-interferometer-based servo writer. These servo writers are relatively expensive in relation to other capital equipment needed to assemble and format disk drives. The more rapidly the servo-writing process is carried out, the less time will be consumed at the servo writer station during manufacturing, with a reduction in the number of servo writer stations needed for mass production and a resultant reduction in the cost burden or overhead associated with each disk drive. Ideally, as few passes for each track as possible should be made at the servo writer station to write the servo patterns for each track.
It is known with a disk drive how to extract and correct for RRO. One example of a disk drive apparatus including an on-board triggered digital sampling analyzer capable of extracting traces and correcting for once-around repeatable runout is found in commonly assigned U.S. Pat. No. 5,444,583 to Ehrlich et al, the disclosure thereof being incorporated herein by reference. An earlier U.S. Pat. No. 3,881,184 to Koepcke et al., entitled: "Adaptive Digital Servo System", described a head position servo control apparatus and method for extracting, processing, storing, and applying feed-forward correction values derived from fundamental frequency and the first eight harmonics of RRO of a disk spindle. The Koepcke et al. patent is also incorporated herein by reference.
As noted above, one known way to measure written-in servo pattern errors is to employ the servo writer to measure these errors immediately after writing of the servo burst patterns and before the drive head-disk assembly (HDA) is removed from the servo writer. Before the HDA leaves the servo writer, the fine position servo bursts are measured by the servo writer, and the amount of any written-in error is digitally recorded in a synchronous manner as a servo correction number (SCN) in a reserved field located near the servo bursts. Later, during disk drive data storage and retrieval operations, the SCN is read and fed into the digital head position servo control loop as a correction to, and thereby cancel, written-in servo position errors. Commonly assigned U.S. Pat. No. 5,237,574 to Weng, entitled: "Error-Resilient Information Encoding", describes a method for detecting and correcting errors in data block headers, including a SCN field. This patent is incorporated by reference.
One of the advantages of the SCN approach is that after the servo writer has written the burst patterns and has developed the SCNs, later passes are used to write the remainder fields of each servo wedge, including the SCNs, in a phase coherent manner. With a single phase coherent writing episode at the servo writer, a relatively smaller error correction code (ECC) or error detection code (EDC) effort is required to protect the SCN fields, and this reduced effort results in a smaller, and therefore more efficient, servo sector layout. However, this prior procedure increases the time each HDA spends on the servo writer, and additional servo writer time represents additional manufacturing costs for the HDA.
Following servo writing, the HDA may be combined with a suitable digital electronics board to complete the disk drive assembly procedure. Once assembled, the disk drive is conventionally placed within a test chamber where it is operated under a variety of stressful operating conditions, such as elevated and/or reduced temperature, and elevated altitude (low ambient pressure), for example. These chambers are frequently referred to as drive "self-scan" test chambers. Most drive failures, if they are likely to occur, will occur during self-scan within one of these chambers, very early in the operational life of the drive. Since the time the drive spends in self-scan is much less costly than time spent at the servo writer station, it would be preferable to measure and record written-in servo burst errors during drive test within the test chamber. However, in drive test, the drive is servoing upon the same data that it would have to measure, with a clear consequence that the position offsets measured from the relative burst amplitudes are a result of the response of the servo system to the written-in errors and not the same as the written-in or "raw" offsets which are measurable at the servo writer. Accordingly, a hitherto unsolved need has arisen for an improved method for post servo writer correction of written-in servo pattern errors within a disk drive.